There are plenty of dry places
around the world, but not every one qualifies as a
desert. Hi, Im Dave Thurlow from the Mount
Washington Observatory for The Weather Notebook. A
desert, to qualify as a true desert, has to get less than ten
inches of rain in a typical year. Even the windblown
Great Plains usually get more than that.

The major deserts of North
America are in the southwest U.S. and Mexico, clustered
around 30 degrees north latitude. Take a look around
the globe, and youll find that deserts seem to like
this latitude. The Sahara in Africa, the Great Indian
Desert, and the Saudi Arabian desert all lie about a third of
the way from the equator to the North Pole. The same
rule applies at 30 degrees south. So why do deserts
like to hang out at a certain distance from the
equator? It starts with simple physics. The
equator gets more sunshine, therefore, more heat than the
poles, so air rises near the equator and sinks near the
poles. Sinking air dries things out and in fact, the
North and South Poles are both dry enough to qualify as
deserts.

But the world is a complex and
puzzling place that spins and curves. Because of this,
you actually have rising air near the equator, then sinking
near 30 degrees north and south, rising near 60 degrees north
and south, and sinking air at the poles. Now, the
upshot of all this rising and sinking is that the
earths circulation causes air to sink, thus get warmer
and drier right where we find the worlds great
deserts. Its no coincidence. And those
deserts are found in between the soggy equator and the stormy
midlatitudes. The Weather Notebook today is written by
Bob Henson. Our show is underwritten by Subaru, the
beauty of all wheel drive with major funding provided by the
National Science Foundation.